This invention relates generally to positive displacement dispensing apparatuses for dispensing precise quantities of a liquid product less than one mm3 in size. More particularly, the present invention relates to a dispensing apparatus having a least two input channels enabling the dispensing of multi-component liquids utilizing a feed screw to both mix the components and dispense the liquid product.
The ability to dispense a minute, precise quantity of liquid such as an adhesive, a conductive epoxy, a two part adhesive, or a solder paste at precise locations on a surface is critical to a number of manufacturing processes, especially in the electronics industry. The assembly of circuit boards, hard disk drives, inkjet cartridges, and flat panel displays are just a few examples. During normal operation, it is important to both achieve and maintain high repeatability in the dispensing quantity inspite of variations in temperature, viscosity, or both.
For some applications, the liquid dispensed is extremely sensitive to such changes, this is especially true where the dispensed liquid has a relatively high viscosity which itself varies as the temperature changes. This can result in changes in the volume of material dispensed over time. An example of this type of problem is in the encapsulation of integrated circuits where typically a two-part epoxy is premixed by the epoxy manufacturer and frozen. The premixed epoxy then must be shipped and then stored in this frozen state. When the buyer is ready to utilize the epoxy it must first be thawed and then used typically within a few days, and in some instances within several hours. Thus, during normal operation the viscosity will change, both due to temperature variation as well as the two components reacting creating variation in dispensed volume over time. This is true especially for those dispensers which utilize pneumatically actuated time/pressure dispensing mechanisms.
In addition, there are also problems relating to the entrapment of air within the liquid to be dispensed because small gas bubbles in the liquid compress, causing sputtering and inaccuracies in the volume of material dispensed. Another problem is the constant almost continuous use that these dispensers can experience when operated under typical conditions on a high volume assembly line. If the material being dispensed hardens or degrades then the valve has to be cleaned and this can be a difficult operation, sometimes requiring the dispensing system to be return to the supplier for reconditioning which results either in higher cost requiring additional systems on hand, or else down time of the assembly line. The ability to rapidly and easily replace those portions of the dispenser which come in contact with the dispensing liquid is very advantageous.
Current dispenser technology for adhesives that are packaged as two parts (ie. resin and hardner for two part epoxies) typically utilize static mixing to blend the resin and hardner together and then dispense the mixture directly to the bondline (i.e. onto the surface desired). A static mixer consists of immovable blades in a short cylindrical tube that facilitate dispersive mixing of the two parts as they exit there respective reservoirs. This technology works well for dispense rates in the milliliter to liter per second range typically used in the automotive and aerospace industries in which the accuracy of dispensed volume is not as critical due to part dimension constrains.
For dispense rates in the microliter per second range typically used in electronic and semiconductor manufacturing, the dispense accuracy is achieved using positive displacement dispenser technology. For this type of dispenser, to obtain the desired accuracies required one uses a single feed screw to convey the adhesive in a known volume for each turn of the feed screw. Thus, the dispensed volume can be accurately controlled by the feed screw motor speed. For systems that use a static mixer, the control typically is pneumatic pressure pushing the adhesive through the mixer. Due to the viscoelastic behavior of most adhesives, controlling the dispense rate and dispense end point when dispensing a bead is difficult. Static mixers can deliver flow rates in the microliter per second range, but typically not with the same accuracy as a positive displacement type pump. Currently the ability to utilize positive displacement pump technology for adhesives that are packaged as two parts, typically requires the addition of a static mixture to blend the resin and hardner together and then feed the blended mixture through a single input channel for dispensing by the feed screw of the positive displacement pump. Thus, there is a need to both blend the resin and hardner together and dispense the adhesive utilizing a single dispensing mechanism.